How do I successfully flip Yu-gi-oh and Pokemon cards on eBay?

When I was a kid I always loved Pokemon and Yugioh cards. The problem was some were really expensive and others essentially worth nothing. There was nothing more exciting then opening up a new booster pack and seeing an ultra rare card worth more then $50 in the shop. A few times I even sold the one card from the pack to buy 3-4 more packs to try to get more rare cards. Now that Im older im looking into buying and reselling the cards for profit. Can anyone make a Hub with a detailed guide on how to do this successfully?